The present invention relates to a covering sleeve for cable joints made of compounds of cross-linked polymeric material which can be applied to several different cables having different outer diameters.
In order to provide a junction between electric cable lengths for carrying electrical energy, particularly in the field of medium and high voltages, the cable conductors are uncovered at the ends, that is, devoid of the respective insulating coatings and, if included, the semiconductive coatings, for the purpose of exposing the conductors thereby allowing their mutual connection. Subsequently, the area without the insulating coating is filled with appropriate materials and then covered with an outer covering so as to restore the required insulating characteristics in the junction area.
For the purpose, a tubular covering element, hereinafter referred to as a sleeve, is fitted over the junction area. The sleeve is made of cross-linked polymeric material consisting of several layers each having specific electrical features, and as a whole, the sleeve is designed to be elastically clamped around the surface of the insulating layer of the connected cables covering the insulating layer itself over a length thereof in the areas contiguous to the conductor junction.
Therefore, the sleeve is radially expanded and maintained under expanded conditions until it is put over the cable junction area. After the sleeve has been put in place, its shrinkage around the cable surface is carried out so that it exerts a pressure thereon capable of ensuring the necessary electric requirements.
In order to keep the sleeve under expanded conditions, it can be made of thermoshrinkable material, i.e., a material which is capable of maintaining the expansion it has received until its shrinkage by heat is caused. However, this technique requires the accomplishment of delicate operations on site for the installation of the sleeve because heating means, such as free flames, are needed in order to achieve the shrinkage of the sleeve itself.
Alternatively, the sleeve may be made of an elastic material and fitted under expanded conditions around a tubular support body which is then removed after the sleeve has been brought to the intended position around the cable junction area, which enables the sleeve to elastically shrink and be clamped around the cable insulating layer.
However, the polymeric materials to be used for the manufacture of sleeves, in which each layer needs particular electrical features well known in the field, generally exhibit, on the discontinuance of the mechanical deformation stress held for a certain period of time, an incomplete elastic return to the original size, that is, the sleeve has an initial temporary residual deformation the degree of which depends, among other things, upon the value of the previously imposed deformation and the temperature and time of stay in the deformed states. Said residual deformation decreases as time goes by and tends to become zero after a certain lapse of time, in the range of some days or months at room temperature (.ltoreq.30.degree. C.).
Due to the fact that after the sleeve has been fitted on the cable junction, it is impossible to wait for a time sufficient to achieve the size recovery which is necessary to the correct clamping of the sleeve, the problem arises of providing a sleeve which, after being stored under expanded conditions, can still be efficiently clamped around the cables by virtue of its elastic features only.
Materials having particular properties of reduced residual deformation could be used for manufacturing the sleeve, but such materials are of difficult formulation because the mechanical characteristics required of them are not normally accompanied by the necessary electrical properties for the different layers so that it becomes difficult to manufacture a sleeve wherein all layers have an elastic behavior exhibiting a reduced residual deformation.
Taking into account the above problem, sleeves of the type described in said patent application Ser. No. 07/464,370 have been manufactured, in which the use of a single sleeve size has been provided for covering cable junctions having different sizes through the employment of covering elements made at the factory and kept in an expanded condition until they are applied.
In said patent application, the problem of providing the sleeve with sufficient expansion to enable it to be fitted over the cable of the greatest diameter in the group of the intended sizes while at the same time enabling it to be efficiently clamped also around the cables of the smallest size in the group has been solved by adopting for the radially outermost sleeve layer only, a material which exhibits a reduced residual deformation on discontinuance of the applied expansion stress, which material is therefore adapted to act on the underlaying layers so that the whole sleeve can efficiently be clamped around the smallest cables in the intended range of sizes.
However, in order to be able to perform its function in an efficient manner, the outer layer must develop an elastic force sufficient to impose the desired shrinkage to the underlying layers, and therefore, it must have a particularly high modulus of elasticity and in addition its thickness must be higher than it would be necessary if only the desired electrical performance of the layer were involved. Furthermore, the material forming the outer layer appears to be comparatively less stressed in the expanded state, relative to the material of the inner layers, for which the use of materials possessing more reduced mechanical qualities is dictated.